Coatings can be deposited on a substrate by techniques that utilize solutions, liquids, vapors, and solids as sources of the deposition materials. Deposition from the vapor phase is commonly associated with the production of a film of material. This production normally takes place on a heated substrate and in a vacuum. However, since it is frequently undesirable to significantly raise the temperature of a substrate, the high temperatures associated with vapor transport make it unsuitable for a number of applications.
Vapor deposition involves three basic steps in the formation of a coating film on a substrate: synthesis or creation of the deposition species, transport of these species from the source to the substrate, and growth of the film on the substrate. These steps can operate independently or interdependently, depending on the particular process. It is preferable to use a process where the steps operate independently, thereby allowing greater flexibility and control.
Chemical vapor deposition (CVD) is a common deposition technique. It offers good quality and excellent uniformity of the deposited film, but requires a relatively high deposition temperature.
In basic thermal chemical vapor deposition process, the reactants flow over a heated substrate surface to deposit a film. Generally the kinetics of the process are dependent on diffusion through the boundary layer between the substrate and the bulk gas-flow region. Temperatures for reactions used in CVD are usually in the range of 500.degree.-1200.degree. C. (930.degree.-2200.degree. F.).
Physical vapor deposition (PVD) is another thin film deposition technique, wherein the material to be deposited is derived from a source by physical means, and then deposited on a substrate.
The two basic processes for physical vapor deposition are: evaporation deposition and sputter deposition. In evaporation, thermal energy converts a solid or target material to the vapor phase. In sputtering, the target is based to a negative potential and bombarded by positive ions of the working gas from the plasma, which knock out the target atoms and convert them to vapor by momentum transfer.
Plasma assisted techniques are, essentially, variants of chemical and physical vapor deposition processes, which rely on the vapor transport of materials to construct new surface. Plasma assisted CVD is similar to thermal CVD with the addition of a radio-frequency biased parallel plate above the substrates. The presence of the plasma activates the deposition reaction. Various compounds are deposited by plasma-assisted chemical vapor deposition (e.g., silicon, carbon, polymers, silicon nitride, iron oxide, silicon carbide). Plasma assisted deposition techniques result in variability in the composition of the deposited species.
In physical vapor deposition, the step of transportation of the species from source to substrate may include the presence of plasma.
Most vapor deposition processes are characterized by relatively high temperature operation, which can be problematic. Plasma assisted deposition can result in undesirable space charge build-up effects. Also, in many applications, precise control of the deposited substrate is lacking with plasma assisted deposition.
It is among the objects of the present invention to provide an improved vapor deposition technique that addresses and solves these and other problems of prior art vapor deposition methods.